Regarding propagation modes in optical waveguides, when the mode numbers n for polarized waves orthogonal to each other are numbered to be 0, 1, 2, . . . in descending order of the effective refractive index, the propagation mode with n=0 is referred to as a basic mode, and other modes are referred to as nth-order modes in proportion to the respective mode numbers.
In addition, modes with are collectively referred to as high-order modes.
In recent years, a Si optical waveguide, in which silica (SiO2) is used for a cladding and silicon (Si) is used for a core, has been attracting attention and anticipation since the size of the optical waveguide can be decreased using a large refractive index difference (Si/SiO2) and the optical waveguide can be manufactured at a relatively low cost using the existing manufacturing facilities for Si large-scale integrated (LSC) circuits.
In an optical waveguide, a Mach-Zehnder (MZ) optical modulator is constituted using an optical coupler/splitter such as a multi-mode interferometer (MMI)-type coupler/splitter or a Y-type coupler/splitter. The MZ optical modulator is disposed along an MZ waveguide and includes a modulating electrode that applies voltage. The MZ optical modulator alters the optical phase between branched waveguides (arms) of the MZ waveguide using the voltage applied by the modulating electrode and turns a light ON/OFF using the interference phenomenon in a coupler on the ejection side. In a case in which two lights in the basic mode are input to the coupler in phase, the coupled light of the two lights is also in the basic mode, and the light is guided to an output waveguide (ON state). On the other hand, in a case in which two lights are in opposite phases, the coupled light is in a high-order mode. In an ordinary MZ optical modulator, since the width of the waveguide is set so that only lights in the basic mode are guided, the coupled light is radiated outside from the waveguide (OFF state).
In the MZ optical modulator, there is a problem in that lights in a radiation mode generated in the coupler propagate through a substrate and couple with lights in a waveguide mode and thus the extinction ratio deteriorates. Therefore, methods for splitting and removing (for example PTL 1 and 2) or blocking (for example, PTL 3) the radiation-mode lights are known.
Furthermore, even in the splitter in the MZ optical modulator, when a high-order-mode light is mixed in, the branching ratio deteriorates and thus the extinction ratio deteriorates. In order to solve this problem, as a method for preventing high-order-mode lights from being mixed into the splitter, PTL 4 discloses that, in a waveguide made of LiNbO3 or the like, high-order-mode lights are removed by decreasing the width of the waveguide in front of the splitter so as to decrease the effective refractive index. PTL 5 discloses that, with an assumption of silica-based glass waveguides, lights in high-order modes are removed from the main waveguide by disposing a subsidiary waveguide having a tapered structure along the main waveguide and using adiabatic transition.
As one of the related arts regarding Si/SiO2 waveguides, NPL 1 (Sections 2.2 and 3.2, FIGS. 1 and 4, and the like) discloses that polarization modes can be split at a length of approximately 10 μm using a polarization splitter (PS) in which a directional coupler (DC) made up of two Si waveguides having a thickness of 200 nm, a width of 400 nm, and a gap of 480 nm is used.